A conventional output buffer with process and temperature compensation, with reference to U.S. Pat. No. 6,538,464 and FIG. 12, the output buffer 50 comprises a temperature detector 51, a voltage detector 52, a compensation circuit 53, a drive circuit 54 and a pre-driver circuit 55. The temperature detector 51 means for detecting temperature from the output buffer 50, the voltage detector 52 means for detecting supply voltage from the output buffer 50, the compensation circuit 53 is capable of generating an impedance control signal to control an impedance compensation circuit 541 of the drive circuit 54. The impedance compensation circuit 541 means for modulating temperature and supply voltage of the output buffer 50. The pre-driver circuit 55 means for controlling slew rate of the drive circuit 54 in response to temperature and supply voltage of the output buffer 50. However, the conventional output buffer 50 merely obtains slew rate information from effects in response to temperature and supply voltage and unable to obtain slew rate information from effects of PMOS transistors and NMOS transistors under various temperature and process conditions.